Froth flotation is a process used to selectively separate hydrophobic from hydrophilic materials. While applicable in a number of industries, one area of great importance is the mining industry where it is commonly used for the recovery of metal sulfide ores or coal, for example. The key steps in the process involve initially grinding the ore (a process known as ‘comminution’) to a relatively fine particle size in order to ‘liberate’ the valuable mineral from the remaining commercially worthless material (known as ‘gangue’). Following this step, the ground ore is mixed with water to form a ‘slurry’ and a surfactant (or ‘collector’) is commonly added to bind to the surface of the desired mineral to render it hydrophobic. The slurry is introduced into an aerated flotation cell (often mechanically agitated with an impeller) such that the hydrophobic particles collide and bind with the air bubbles and rise to the surface of the cell, forming a froth of purified mineral which is then collected. The recovery of a mineral exhibits a strong dependence on the size of the particles, with an approximate upper limit being in the range of ten to one hundred microns (10-100 μm). Relatively small particles do not have enough inertia to deviate from the streamlines around an air bubble and collide with it. Detachment from the bubble can occur as a result of the shear forces within the turbulent flow in the case of relatively large particles or the gravitational force acting on the coarser particles exceeding the buoyancy and/or bubble-particle attraction forces. Despite the success of the froth flotation process, the costs associated with grinding an ore to sub-micron size are extremely high and require high energy consumption. It has been reported that an approximate thirty-seven percent (37%) in energy savings could be gained if the upper limit of the particle size could be increased to one millimeter (1 mm), but this would require a means for preventing detachment from the air bubbles.
One approach for enhancing coarse particle recovery is the use of very small bubbles in the slurry, often referred to as ‘picobubbles’ or ‘nanobubbles’ due to their nanometer-scale length dimensions. Nano-bubbles form naturally or can be generated artificially. It has been reported that that the presence of nanobubbles can improve recovery by five to fifty percent (5-50%). After the nano/micro-bubbles are injected, they attach to particles or stay dispersed in the solution.
Current particles recovery technologies are limited currently at about one hundred microns (100 μm) for minerals and six hundred (600 μm) for coal. Enabling coarse flotation has the potential to reduces the energy in the process chain with significant economical benefits.